This invention pertains to a method of preparing an oxidation catalyst containing gold and titanium.
Catalysts containing gold and titanium are useful in the hydro-oxidation of olefins to olefin oxides. For example, it is known to oxidize propylene with oxygen in the presence of hydrogen and a catalyst containing gold and titanium to produce propylene oxide. Propylene oxide is a commercially important raw material for the production of propylene glycols and polyether polyols which are used in preparing polyurethanes.
The catalyst used in the aforementioned hydro-oxidation process more specifically comprises gold on a titanium-containing support. The support may be selected, for example, from titanosilicates, titanium dioxide, titanium dispersed on silica, and certain metal titanates. Optionally, the catalyst can further contain a promoter metal, such as an alkali, alkaline earth, or lanthanide rare earth metal, for the purpose of enhancing catalytic performance. Prior art representative of this process and catalyst composition includes PCT patent publications WO 98/00413, WO 98/00414, and WO 98/00415.
It is known to prepare a hydro-oxidation catalyst comprising a platinum group metal on a titano silicate support by impregnation methods. Art of this type, represented by PCT patent publication WO 96/02323, discloses impregnating a solution of a platinum group metal salt onto a titanosilicate support, and thereafter reducing the impregnated support under hydrogen to adjust the bond energy states of the platinum group metal. Reduction under hydrogen disadvantageously may require high temperatures, and the reduction cannot be controlled sufficiently well.
Other art, such as EP-A1-0,709,360, teaches catalysts comprising ultra-fine particles of gold deposited on titanium dioxide being prepared by a deposition precipitation method. This method involves preparing an aqueous solution of a soluble gold salt, adjusting the pH to between 7 and 11, and then adding titanium dioxide to the solution. The resulting composite is calcined to obtain ultra-fine elemental gold particles deposited on the titanium dioxide carrier.
An alternative, but related method, exemplified in U.S. Pat. No. 4,839,327 and EP-A1-0709,360, involves a coprecipitation method. Here, an aqueous gold solution with a pH value of between 7 and 11 is added dropwise to an aqueous solution of a soluble titanium salt adjusted to the same pH range, so as to form a coprecipitate. The coprecipitate is calcined to obtain metallic gold deposited on titanium dioxide.
In another deposition precipitation method, exemplified by U.S. Pat. No. 4,937,219, a catalyst comprising ultra-fine gold particles immobilized on a mixed alkaline earth-titanium oxide is prepared. The preparation involves dissolving or suspending an alkaline earth-titanium compound, such as strontium titanate, in an aqueous solution of a gold compound, adjusting the pH to between 7 and 11, and adding dropwise a reducing agent thereby causing ultra-fine gold particles to be precipitated onto the alkaline earth titanate. The reducing agent is disclosed to be formalin, hydrazine, or citrate salts. A variation of this method is found in U.S. Pat. No. 5,051,394, wherein the pH of an aqueous solution containing a gold compound and a water-soluble titanium salt is adjusted with an alkali compound to yield a coprecipitate, to which is added a carboxylic acid or salt thereof. The coprecipitate thus treated is heated to form the catalyst comprising metallic gold deposited on titanium oxide.
All of the aforementioned deposition precipitation and coprecipitation methods suffer from multiple disadvantages. Specifically, the methods of the prior art require the accurate control of deposition conditions over a long period of time. Moreover, when a reductant is used, the gold particles may be reduced in solution before adhering to the support, which results in an inefficient use of gold. Since there is poor control over the precise amount of gold which is deposited onto the support, additional efforts are required to recover unused gold from the deposition solution. Even more disadvantageously, the prior art methods are temperature sensitive. They also require the use of large quantities of solvents and control of pH. Finally, the prior art methods may result in poor adherence of the gold particles onto the support.
In view of the above, it would be desirable to discover a simple, efficient, and reproducible method of preparing an active oxidation catalyst comprising gold deposited on a titanium-containing support. It would be desirable if the process avoided the disadvantages of the deposition precipitation and coprecipitation techniques. It would be more desirable if the process could be adapted to practical forms of the catalyst, such as, pelleted and extruded titanium-containing supports. It would be even more desirable if the process did not require a gold recovery stage. Such a process would advantageously reduce catalyst preparation efforts and costs, and equally importantly, conserve gold.